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The Cadmium Zinc Telluride (CZT) Detector Array Innovation Pathway

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The Cadmium Zinc Telluride (CZT) Detector Array Innovation Pathway The Cadmium Zinc Telluride (CZT) Detector Array Innovation Pathway This pathway describes the development of soft gamma-ray/hard x-ray detector arrays at NASA’s Goddard Space Flight Center (GSFC, or Goddard), through a collaboration between the gamma-ray spectroscopy group and the detector branch. The Cadmium Zinc Telluride (CdZnTe, or CZT) detectors which resulted from this decade-long development were first flown on the wildly successful SWIFT mission in 2004, and have since enabled a whole class of imaging applications in the hard X-ray/soft gamma-ray range. The CZT detectors filled a long recognized need for a room temperature semiconductor capable of high resolution imaging and spectroscopy in this particular waveband. Gestation Period (pre-1993 collaboration) Although the Goddard gamma-ray spectroscopy group and the detector branch didn’t begin collaborating on a CZT development program until 1993, relevant roots of the innovation pathway trace much farther back. In the early 1990s, there were very few instruments covering the hard x-ray / low-energy gamma-ray spectral band (5-500 keV) in existence. This gap was due more to the technical challenges associated with making detectors in this range, than a lack of scientific interest; however challenges of the former limited numbers of the latter [I73]. The Goddard scientists were hoping to fill that gap by branching out into this new energy band, which they believed held the key to understanding such diverse phenomena as: • element creation, explosion dynamics and event rates for supernovae • the origin of gamma-ray bursts through sensitive searches for cyclotron and other lines • physical conditions in the vicinity of neutron star surfaces through observations of cyclotron and other lines in x-ray pulsars • physical conditions in the central engines of AGN [D1] At the time, the default hard X-ray spectroscopy detectors were Ge:Li (Germanium doped with Lithium). These detectors had relatively low Z (which meant that the detectors would need to be very thick to create sufficient stopping power at the relevant energy band) and needed to be cryogenically cooled (requiring the accompaniment of the mass and complexity associated with advanced cooling apparatus, effectively rendering explorer class missions out of reach) [I52, 53]. Recognizing that suitable detector technology was a prerequisite to achieving their objectives, the group had been “on the lookout” [I73] for a suitable detector for some time. Being “on the lookout” in this context involved regular attendance at the conferences where researchers presented new developments in the realm of high Z, room-temperature solid state detectors [I73]. By the late 1980s, the scientists began to hear about two new semiconductor compounds – Mercuric Iodide (Hg I2) and Cadmium Telluride (CdTe) – which, following nearly two decades of research since their discovery in 1970/1971 were beginning to show promise as radiation detectors [D37, I80]. The former development had been heavily funded by the Department of Energy in the context of nuclear monitoring [D1]. The latter was under development by smaller companies interested in applications to medical imaging [I80]. Both materials offered the promise of room-temperature semiconductor gamma-ray detection, but each presented © Zoe Szajnfarber 2011, Massachusetts Institute of Technology, [email protected] Page 1 significant technical challenges which limited their practical utility. By the late 80s, when the scientists were monitoring developments, HgI2 seemed the leading alternative [D8, D1]. Thus, in 1991, the scientists put in an SR&T (Sustaining Research and Technology, now called NASA Research Activity or NRA) proposal to study the application of HgI2 devices at NASA. They won the SR&T bid as well as a follow-on grant to investigate the relative detector performance characteristics of different state-of-the-art detectors using balloon-based platforms (including the cryo-cooled baseline Ge:Li, room temperature HgI2 and alloys of CdTe and Zn) [D1, I52]. For the second grant, a young astrophysicist (CSA#4) was hired as an NRC post doc, specifically to investigate room temperature gamma-ray detectors. It was during preparation of that proposal that the Director of Space Sciences (CSA#12) recommended that the scientists collaborate with Goddard’s detector branch and made the introductions. The director had learned the value of this type of interdisciplinary collaboration through prior work with the detector branch head (primarily in the context of microcalorimeter development for AXAF (Advanced X- ray Astrophysics Facility) – see microcalorimeter case). Project Initiation (A phone call and an executive decision) In a sense, NASA’s decade-long development of CdZnTe gamma-ray detector technology was initiated by a phone call from above. One afternoon in 1993, Goddard’s detector branch head got an unusual call. It was from the Director of Space Science at Goddard. His request was for the detector branch to support the high energy physics group in developing room temperature gamma-ray detectors, an area of science he believed would be critically important in the future. Based on preliminary investigation by Goddard’s science community, two potential candidate detector materials seemed promising: HgI2 and CdZnTe. The director expressed no preference between them. As recalls the branch head (BH#2), “this was not a guy you said no to.” So, following the conversation, the branch head returned to his desk and “did his homework” on the options [I59]. Interestingly, the Director of Space Science doesn’t remember that conversation as having been particularly unusual or important [I74]. While it wasn’t common for a science director to call an engineering branch head, it was an important part of what the director saw as his job. As he viewed the situation: “either I could give equal, uniform blanket approval to everything that came out of my organization; or I could demonstrate some preference for some things that I thought were more promising.” He chose the latter approach realizing that “I could be much more effective if I occasionally (not all the time) demonstrated passion (or anger)… this was one case where [BH#2] was being pulled [in other directions] so I thought it was important to let him know how important I thought this was” [I74]. Although the director had no formal influence over the engineering directorate, he found that these “demonstrated preferences” served to guide the level of effort, provided by engineering to support proposals, and projects. This case was no different. Within a couple of weeks of “homework”, the branch head had (independently) come to the conclusion that CZT was the most viable path forward [I59]. In the time since the scientists’ assessment of the state of the field, a new material growing process had been developed for undoped CdTe as well as the invention of a new semiconductor compound CdZnTe [D9, D10]. These two advances changed the relative assessment of the device options. Based on the updated information, the branch head chose CZT for two reasons. First, HgI2 is a © Zoe Szajnfarber 2011, Massachusetts Institute of Technology, [email protected] Page 2 “miserable material to work with,” [I59] particularly from a health and safety point of view. To work with it in their fabrication lab, many expensive changes would have had to have been made. CZT didn’t pose these concerns. Second, the main group working on HgI2 was located in Israel. While this did not preclude collaboration, it certainly would have made it more challenging. Expertises in CZT processing were domestic. Combined, these factors made the branch head’s decision to pursue CZT detectors a straightforward one [I59]. Thus, a few short months into NASA’s development, a major potential technological trajectory had already been pruned. HgI2 was pursued in the broader community and was later found to have other performance-oriented challenges compared to CZT/CdTe (which have since emerged as the dominant technologies) [I48]. Goal Guided Exploration Almost from the beginning, the science goal of finding the origin of Gamma Ray Bursts guided the technology development. GRBs are the most luminous electromagnetic events occurring in the universe, and for several decades represented a major mystery in the field of high energy astrophysics. Originally discovered by a satellite designed to monitor for covert nuclear weapons tests in space in the 1960s,1 and declassified in 1973, these gamma-ray bursts of “cosmic origin” became a topic of much scientific speculation. No one knew what they were or where they were coming from. Even accurately positioning these short-lived (~1 second) high energy bursts, which seemed to crop up at random locations, posed a major challenge: Since, unlike light at less-energetic wave-lengths, gamma-rays cannot be bent (and focused), the detector area needs to be as large as the desired imaging area [D39]. Also, since scientists were unable to predict where the next burst would come from, this imaging area needed to be wide [D29]. This science need translated into a requirement for a wide area detector plane with sub 100 µm position resolution [D13]. The intention was to procure CZT wafers from industry; have the detector branch pattern fine contact structures, wire bond leads and package the detectors as a suitably large array, per the design of the science team, as shown in Figure Error! No text of specified style in document.-1. However, in order to accomplish this, an interrelated
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